The use of multichip modules for mounting and interconnecting a plurality of integrated circuit ("IC") chips is becoming increasingly important and common as electronic devices continue to shrink in size and operate at higher frequencies. In such modules, chips are mounted on one or more chip-carrying substrates. The substrates are typically multilayered and contain the various signal, power and ground lines needed by the chips to operate. By mounting many chips in a multichip module, it is possible to achieve very high chip densities, thereby minimizing the lengths of the signal paths between chips, and allowing a large number of chips to be contained in a relatively small space. In order to realize these benefits, however, the multilayered substrates that are used have become quite complex. In one common method of construction, such multilayered substrates often include a plurality of dielectric layers of a polyimide material between patterned metal layers.
When constructed, a multichip module may contain many complex and highly integrated active semiconductor devices, and passive devices such as bypass capacitors and resistors, in addition to the necessary electrical contacts, traces and other structures used for supplying power and ground to the devices and for communications among the devices and between the module and the outside. Many of these structures are susceptible to failure when exposed to reactive gaseous contaminants such as water vapor. In this regard, it is noted that the polyimide layers frequently used in multichip modules are apt to adsorb and then outgas water vapor.
One prior art approach to preventing chips and other devices or structures from being exposed to potentially deleterious gases is encapsulation. While this approach may be somewhat useful in conjunction with single-chip packages which generally do not employ multilayered substrates, it is not as useful with complex multichip modules. Unlike single-chip modules, in multichip modules, it is often desirable to be able to repair or replace individual components within the module without replacing the whole module. This is generally quite difficult or impossible if the entire module is encapsulated. Moreover, encapsulation would not prevent water vapor trapped within the encapsulated structure from migrating to sensitive components.
Another approach, used both with single chip and multichip modules, is to place the sensitive components in a hermetically sealed chamber which is flushed and filled with a chemically inert gas such as nitrogen or helium. This approach has two problems. First, it is common for such packages to eventually develop leaks due to small cracks or holes, thereby allowing the inert gas to leak out and deleterious gases to leak in. In this regard, it is noted that chip packages may be subject to repeated thermal cycling over a substantial temperature range, which creates substantial mechanical stresses on the seals and leads to the development of cracks. Second, contaminants from the package itself can leak into the sealed chamber and cause damage. For example, water vapor from polyimide layers or other surfaces can outgas into the sealed chamber, where it is then trapped. Thus, moisture may enter into a micro-electronic circuit either from an external leak or by outgassing from the internal components. Outgassing of water vapor is a particularly difficult problem since water, because of its bipolar nature, readily adheres to many surfaces when they are exposed to normal atmospheric conditions where significant moisture is likely to be present. The adsorbed water vapor then slowly outgasses when surfaces are placed within a sealed, moisture-free enclosure.
In some electronic applications, such as in the construction of vacuum tubes, this problem is solved by a "bake-out" of the exposed surfaces to drive off adsorbed gases prior to sealing. However, this approach is not very useful with micro-electronic devices, such as semiconductor chips and the like, which may be destroyed if heated to too high a temperature. While IC chips are typically heated to drive off moisture, the heating temperature is lowers and the duration of heating is generally shorter than that used with less sensitive components such as vacuum tubes. Likewise, elevating a chip module to a temperature sufficiently high to completely drive off contaminant gases may not be practical in connection with repairs that may be required in the field, where the components may be exposed to atmosphere when the package is opened for repair. Finally, even at high temperature, a bake-out of adsorbed gases takes a substantial time, thereby adding to production costs, or, if done in the field, adding to delay in getting the system back "on-line."
Accordingly, there is a need for an improved apparatuses and methods for packaging micro-electronic components in a multichip module whereby said components are protected from the deleterious effects of reactive gases.